The present invention relates to processes for producing electrical insulating varnishes used for insulating materials or adhesive films for printed circuits boards which can comply with the request for smaller thickness and higher wiring density of such circuit boards on which electronic parts are mounted, and multilayer printed circuit boards made by using such insulating varnishes.
Printed circuit boards are usually produced by forming circuits on copper-clad laminates obtained by laminating copper foils and prepregs and molding the laminates by heating under pressure.
Multilayer printed circuit boards are manufactured by forming circuits on the surfaces of multilayer copper-clad laminates having inner layer circuits formed therein, which laminates have been obtained by hot-pressure laminating said printed circuit boards, or those boards and copper foils, with prepregs interposed therebetween.
As prepregs for printed circuit boards, there have been used glass cloth prepregs obtained by impregnating a resin in glass cloth and drying the resin into a half-curedstate. In manufacture of multilayer printed circuit boards, adhesive films made of half-cured resins having film forming properties, which are prepregs containing no glass cloth, such as disclosed in JP-A-6-200216 and JP-A-6-242465, or adhesive films having a copper foil formed on one side, such as proposed in JP-A-6-196862, have been used beside said glass cloth prepregs.
When the term "film forming properties" is used in this specification, it means the quality of the resin which is unsusceptible to such trouble as cracking or chip-off during transport, cutting, lamination and other works of prepregs and to troubles in the ensuing hot-pressure molding, such as abnormal diminution of the interlayer insulating layers at the areas where the inner layer circuits are present, lowering of interlayer insulation resistance, and short-circuiting.
The recent years have seen sharp acceleration of the tendency to smaller size and weight, higher performance and lower production cost of electronic devices, consequently calling for higher packaging density, smaller thickness, higher reliability and lower cost of the printed circuit boards used in such electronic devices.
For realizing higher density of printed circuit boards, finer wiring is essential, and this requires good surface flatness and high dimensional stability. Formation of fine through-holes and/or interstitial via holes (IVH) and good laser drilling workability are also demanded. For obtaining higher surface flatness, it is necessary to enhance fluidity of the resin during multilayer laminate molding, and this purpose can best be met by use of a thermosetting resin such as epoxy resin.
Epoxy resins, however, show high fluidity because of low molecular weight at the stage before molding and are incapable of forming a sheet-shaped insulating material. Hitherto, therefore, prepregs prepared by impregnating an insulating resin in a reinforcing base material such as glass cloth have been used for forming the insulating layers, but it has become hardly possible for these conventional prepregs to answer to the above requirements.
The glass cloth generally used for prepregs at present has the spaces between the yarns (glass fiber bundles) enlarged proportionally as its thickness is reduced. Therefore, the smaller the thickness of the cloth, the higher becomes the probability of occurrence of a phenomenon in which yarns are bent or the warp and weft, which normally ought to cross at right angles, fail to cross at right angles. This phenomenon tends to cause an abnormal dimensional change or warping after hot pressure molding. Further, the smaller the thickness of glass cloth, the greater are the spaces between the yarns, hence the lower becomes the volume fraction of prepreg fibers, resulting in a reduced rigidity of the interlayer insulating layer. This tends to enlarge dimensional deflection in the steps, such as parts packaging step, after working of outer layer circuits.
The minimal permissible thickness of currently used glass cloth is 30 .mu.m, and the thickness of prepregs using glass cloth of such minimal thickness is about 40 .mu.m. If the resin proportion is reduced for making the prepreg thickness smaller than this, resin filling of the recesses of inner layer circuits is adversely affected, allowing formation of voids. Also, thickness reduction of glass cloth beyond the above limit invites corresponding lowering of strength of the cloth itself, so that the glass cloth becomes liable to break in the step of immersion of the glass cloth in the resin, making it difficult to produce the desired prepregs. Another problem of the multilayer printed circuit boards made by using these glass cloth-based prepregs is that there tends to take place center deflection or lost of core center due to ununiform distribution of glass cloth during fine drilling, resulting in break of the drill. Due to the presence of glass fibers, laser drilling efficiency is low and unevenness of inner layer circuits tends to emerge on the surface to degrade surface flatness of the board. Therefore, use of the presently available glass cloth-based prepregs can not accommodate the mounting requirements for higher packaging density and smaller thickness of the multilayer printed circuit boards.
On the other hand, the adhesive films or copper-foiled adhesive films made of prepregs containing no glass cloth have the advantage of enabling production of the circuit boards with smaller thickness and are also prominent in fine drilling workability, laser drilling efficiency and surface flattening performance. However, the multilayer printed circuit boards made of these prepregs are intolerably low in rigidity because of absence of glass cloth base in the outer insulating layers. This low rigidity is most manifest under high temperatures and causative of positional deflection during packaging of parts. It is also responsible for poor wire bonding workability. Further, because of large thermal expansion coefficient due to the absence of glass cloth in the outer insulating layers, the difference in thermal expansion between the board and the packaged parts is large, hence reliability of connection to the packaged parts is low. Also, cracking or break tends to occur at the solder joints due to thermal expansion or contraction of the board as it is heated or cooled. Thus, with the adhesive films or copper-foiled adhesive films made of the currently used glass cloth-free prepregs, it is hardly possible to comply with the rising request for higher packaging density and smaller size of the multilayer printed circuit boards.
As a new insulating material that can meet the requests for higher packaging density, smaller size, higher reliability and lower cost of multilayer printed circuit boards, which could not be attained with the conventional prepregs, there has been offered a sheet-like insulating material produced by casting a varnish containing no glass cloth and having electrical insulating whiskers dispersed in the insulating resin for shape retention. It was found, however, that this insulating material also had the problems. For instance, for dispersing electrical insulating whiskers in the insulating resin, a specific mixing equipment is required, and it is also essential to conduct an appropriate surface treatement of the electrical insulating whiskers used. It was found that even if these requirements are met, the produced insulating material could still cause improper insulation when it is used for multilayer printed circuit boards.
The electrical insulating whiskers have a tendency to aggregate in the dry state, so that for properly dispersing these whiskers in the insulating resin, it is necessary to use a specific mixing equipment or to apply a proper surface treatment on the whiskers. Even if these requirements could have duly been dealt with, it is still impossible to perfectly eliminate the aggregates of these whiskers.
In the multilayer printed circuit boards, the insulating layers are provided with a greater thickness than the inner layer circuits for ensuring filling of surface unevenness of the inner layer circuits. For reducing the overall thickness of the multilayer printed circuit boards, it is desirable to lessen the insulating layer thickness as much as possible within limits that allow retention of required insulation. In view of this, the insulating layer thickness is usually set to be 25 to 100 .mu.m. Regarding the size (length) of the aggregates of electrical insulating whiskers, when we procured the commercial whiskers by designating their average length to be 30 .mu.m, the procured whiskers included those having a length exceeding 50 .mu.m, some of them being longer than 300 .mu.m. In case a sheet-shaped insulating material using these electrical insulating whiskers is applied to multilayer printed circuit boards, a varnish containing such lengthy electrical insulating whiskers or the aggregates thereof mentioned above is used, so that these whiskers or the aggregates thereof tend to contact each othere between the conductors to cause improper insulation such as observed when using conductive anodic filaments (CAF).